What is the net charge on Phe-Asp at each pH?
a. pH 1
b. pH 7
c. pH 14
To determine the net charge on Phe-Asp at each pH, we need to examine the ionizable amino acid residues present in Phe-Asp and their corresponding pKa values.
Phe-Asp is a dipeptide composed of phenylalanine (Phe) and aspartic acid (Asp). Both amino acids contain ionizable side chains.
Phenylalanine (Phe) does not have an ionizable side chain, so its charge remains neutral regardless of pH.
Aspartic acid (Asp) has a carboxylic acid side chain, which can be deprotonated to become negatively charged when the pH is above its pKa. The pKa value for aspartic acid is approximately 3.9.
Now, let's analyze the net charge on Phe-Asp at each pH:
a. pH 1:
At pH 1, the solution is highly acidic. Since the pKa value of aspartic acid is higher than the pH, the carboxyl group of aspartic acid will remain protonated (neutral). Therefore, the net charge on Phe-Asp at pH 1 will be neutral.
b. pH 7:
At pH 7, the solution is close to neutral. The pKa of aspartic acid is lower than the pH, so the carboxyl group will be deprotonated (negatively charged). However, the amino group of aspartic acid, which has a pKa around 9.9, will still be protonated (neutral). Therefore, the net charge on Phe-Asp at pH 7 will be -1, as the negatively charged carboxyl group dominates.
c. pH 14:
At pH 14, the solution is highly basic. Both the carboxyl group and amino group of aspartic acid will be deprotonated and negatively charged. So, the net charge on Phe-Asp at pH 14 will be -2, as both aspartic acid residues on each end of the dipeptide are negatively charged.
In summary:
- At pH 1, the net charge on Phe-Asp is neutral.
- At pH 7, the net charge on Phe-Asp is -1.
- At pH 14, the net charge on Phe-Asp is -2.